-Patient preference in side effects likely to influence choice of Arimidex or Nolvadex

SAN ANTONIO -- For postmenopausal women with a very early form of breast cancer, the aromatase inhibitors anastrozole (Arimidex) and tamoxifen (Nolvadex) were of similar efficacy in preventing recurrence, researchers said here.

But differences in the side effects probably mean patient preference, rather than a decided gain in efficacy, will govern how the drugs are used, according to results from parallel trials presented at the San Antonio Breast Cancer Symposium (SABCS).

Action Points

Note that this study was published as an abstract and presented at a conference. These data and conclusions should be considered to be preliminary until published in a peer-reviewed journal.

For postmenopausal women with a very early form of breast cancer, the aromatase inhibitor anastrozole and tamoxifen were of similar efficacy in preventing recurrence.

Point out that treatment will be highly affected by how the drugs are tolerated given the similar efficacy.

That's what the trialists hoped would happen, according to Jack Cuzick, PhD, of the Queen Mary University of London, one of the leaders of the IBIS-II study.

Cuzick told MedPage Today the trials were planned at the same time, with discussion among the two research groups, so that the studies are almost identical. The main difference is that NSABP B-35 required participants to have radiation therapy, while only about 70% of participants in IBIS-II had radiation.

IBIS-II involved women from 14 countries outside North America, while NSABP B-35 was conducted in the U.S. and Canada, and the primary goal in both studies was to evaluate differences in recurrence rates.

But a key secondary goal was to look at the side effect profiles of the two drugs, according to Patricia Ganz, MD, of the University of California Los Angeles.

At this meeting, Cuzick focused on the efficacy outcome of IBIS-II, while Ganz focused on patient-reported side effects in a quality of life (QOL) substudy of NASBP B-35, since the efficacy results were reported earlier this year.

It's important to remember, Ganz said, that recurrences are going to be rare in this patient population so that quality of life is an important factor. "These are women with DCIS -- we're not talking about women who are at risk of dying immediately from this disease," she said.

Cuzick told reporters before his oral presentation that IBIS-II had 2,938 evaluable patients randomized to take 1 mg of oral anastrozole or 20 mg of oral tamoxifen daily for 5 years.

Patients were followed for a median of 7.2 years, during which 144 breast cancers recurred, 67 among anastrozole patients and 77 among those taking tamoxifen. The 0.89 hazard ratio in favor of anastrozole was not statistically significant but did establish that the drug was non-inferior to tamoxifen.

All told, 33 women in the anastrozole group died, compared with 36 taking tamoxifen, but the difference was not significant; no specific cause was more common in one group than the other, Cuzick and colleagues found.

The result were slightly different in NSABP B-35, where 3,104 women were randomized to the same drug doses, and the recurrence rate was 27% lower among those taking anastrozole, a result that did reach statistical significance.

Taken together in a meta-analysis, the results suggest about a significant 21% edge for anastrozole in preventing recurrence. "But that could easily be outweighed," he told MedPage Today, "by potential differences in side-effect profiles."

In both trials, the frequency of side effects was similar between the arms, but the types of adverse events differed.

The NSABP B-35 investigators conducted a formal QOL study involving a subset of 1,193 women, Ganz said, which delineated a pattern of patient-reported adverse events that distinguish between the two drugs. Specifically, the investigators found:

No significant differences in physical health scores, mental health scores, energy and fatigue, or symptoms of depression over 5 years.

Vasomotor symptoms, difficulty with bladder control, and gynecological symptoms were significantly more severe in the tamoxifen group.

Musculoskeletal pain and vaginal symptoms were significantly worse in the anastrozole group.

In IBIS-II, Cuzick reported, the number of women reporting any adverse event was similar -- 91% in the anastrozole arm and 93% in the tamoxifen arm -- with more fractures, musculoskeletal events, high cholesterol, and strokes with anastrozole and more muscle spasms, gynecological cancers and symptoms, vasomotor symptoms, and deep vein thrombosis with tamoxifen.

For patients and doctors, choosing which drug to use is going to be a "complicated" balancing act, commented C. Kent Osborne, MD, of Baylor College of Medicine in Houston, who was not part of the studies but who moderated an SABCS.

Such things as a history of thromboembolic events or current osteoporosis will come into play as women choose their therapy, Osborne said, and will likely form the basis for many therapeutic decisions.

One thing the evidence doesn't support, he said, is recommending one drug over another purely on the basis of efficacy.

Last Updated December 14, 2015

The NSABP-B35 trial was supported by the National Cancer Institute and AstraZeneca.

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